Grazing or "skip" interferometry is a known optical metrology technique wherein an optic of interest is tested at a non-normal angle of incidence to a surface of the optic. One drawback to this technique is that as the angle of grazing incidence increases, the foreshortening of an image in an evaluation plane is increased, with a corresponding loss of lateral resolution in the direction of grazing incidence. Unfortunately, the direction of grazing incidence is often the direction wherein the greatest surface measurement accuracy is required.
An advantage of using interferometry over other known grazing incidence metrology techniques, such as fringe scanning or profilometry, is that a complete measurement of the entire surface of the optic of interest is obtained at one time, as opposed to requiring several scans that must later be combined, or "stitched" together. In general, stitching is a process of taking several data scans and subsequently combining the scans to obtain a map of the entire surface of the optic.
A further disadvantage of the conventional fringe scanning technique is a requirement of providing a reference optic of the same geometric form, and better surface quality, than the optic under test. In contradistinction, a grazing incidence interferometer may be set up using only flat and spherical optics, which are generally more readily obtainable and more readily characterized than a special purpose reference optic.
As such, it can be realized that the use of grazing or skip interferometry is preferred in that it eliminates the requirement to stitch together the results of several measurements, and that it also eliminates a requirement of providing, aligning, and maintaining a high-quality, non-standard test optic.
It is thus an object of this invention to provide an interferometric grazing incidence technique that overcomes the problems resulting from a foreshortening of the image, and the corresponding loss of lateral resolution in the direction of grazing incidence.